009767 rock hazard rating sys

8/11/2019 009767 Rock Hazard Rating Sys

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PREFACE

Many miles of highway have adjacent rock slopes that

are subject to rockfall. This potential for rockfall

is due in part to past construction practices that

relied on overly aggressive excavation techniques.

Although these practices facilitated removal of broken

material, they commonly resulted in slopes more prone

to rockfall than necessary.

The Rockfall Hazard Rating System (RHRS) is intended

to be a tool that will allow transportation agencies

to address their rockfall hazards proactively instead

of simply reacting to rockfall accidents. The RHRS

provides a legally defensible, standardized way to

prioritize the use of the limited construction funds

available by numerically differentiating the apparent

risks at rockfall sites.

The Oregon Department of Transportation (ODOT) began

developing the RHRS in 1984. Funding from a Federal

Highway Administration (FHWA) sponsored, pooled-fund,

Highway Planning and Research (HPR) Grant allowed ODOT

to complete development of the system and test it at

more than 3,000 sites.

The workshop at which the RHRS is presented is

intended for the those personnel who will implement

*he RHRS and be responsible for evaluating and rating

the rockfall sites, and for the managers who will

decide whether their agency should adopt the RHRS.

For these managers, the first chapter and hour of the

workshop is an executive summary.

Those participants who will implement and perform the

RHRS will receive two days of training. Their first

day will be spent in a classroom setting, where they

will learn how to perform the ratings, create a

rockfall database, and use that information to set

rockfall remediation priorities. Their last day's

task will be a hands-on field exercise that requires

the participants to apply the RHRS's process at two

actual rockfall sites within the agency's

jurisdiction.


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TABLE OF CONTENTS

Page

CHAPTER 1: EXECUTIVE SUMARY

1.1 Introduction

1.2 Evolution of the Rockfall Hazard Rating System

1.3 Summary of System Features

1.4 RHRS Benefits

1.4.1 Knowledge

1.4.2 Public Perception

1.4.3 Legal Protection

1.5 Implementation

1.5.1 RHRS Modification

1.5.2 Training

1.5.3 costs

1.6 Limitations

1.7 Conclusion

CHAPTER 2: SLOPE SURVEY

2.1 Purpose

2.2 Approach

2.3 Personnel

2.4 Information Gathered

CHAPTER 3: PRELIMINARY RATING

3.1 Purpose

3.2 Criteria

3.2.1 Estimated Potential for Rockfall on Roadway

3.2.2 Historical Rockfall Activity

3.3 Classification Description

3.4 How to Use the Preliminary Rating Results

3.5 Workshop Problem, Classroom Exercise 1

CHAPTER 4: DETAILED RATING

4.1 Purpose

4.2 Overview

4.3 Scoring System

4.4 Scoring Aids

4.4.1 Graphs

4.4.2 Exponent Formula

4.4.3 Scoring Tables

CHAPTER 5: DETAILED RATING CATEGORIES

5.1 Category Narratives

5.2 Category Photographs

5.3 Slope Height Category

5.3.1 Category Significance

5.3.2 Method of Measurement

5.3.3 Criteria Examples

5.3.4 Classroom Exercise 1

5.4 Ditch Effectiveness Category


5.4.2 Category Measurement

5.4.3 Criteria Narratives

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5.5 Average Vehicle Risk (AVR) Category


5.5.2 Criteria Example


5.6 Percent of Decision Sight Distance Category



5.6.3 AASHTO Decision Sight Distances

5.6.4 DSD Formula



5.7 Roadway Width Category





5.8 Geologic Character

5.8.1 Case One, Structural Condition Category

5.8.1.1 Category Significance

5.8.1.2 Criteria Narratives

5.8.1.3 Criteria Examples

5.8.2 Case One, Rock Friction Category




5.8.3 Case Two, Structural Condition Category




5.8.4 Case Two, Difference in Erosion Rates Category




5.8.5 Selecting The Proper Case


5.9 Block Size or Volume of Rockfall Per Event Category


5.9.2 Which Criterion to Use



5.10 Climate and Presence of Water on Slope Category


5.10.2 Method of Evaluation

5.10.3 Information Source

5.10.4 Criteria Example


5.11 Rockfall History Category



5.11.3 Sources of Information


5.12 Summary of Classroom Exercise 1

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5.13

Epilogue

CHAPTER 6: PRELIMINARY DESIGN AND COST ESTIMATE

6.1 Approach to Rockfall Control

6.2 Rockfall Remediation Designs

6.2.1 Common Rockfall Remediation Techniques

6.2.2 Reviewing Preliminary Designs

6.3 Cost Estimate

, 6.4 Classroom Exercise 1

CHAPTER 7: PROJECT IDENTIFICATION AND DEVELOPMENT

7.1 Project Identification

7.1.1 Score Method

7.2.2 Ratio Method

7.2.3 Remedial Approach Method

7.2.4 Proximity Method

7.2 Rockfall Related Accidents

CHAPTER 8: ANNUAL REVIEW AND UPDATE

8.1 Review and Update

8.2 Review Purpose

CHAPTER 9: THE ROCKFALL DATABASE MANAGEMENT ROGRAM

9.1 The Value of an Automated Database (RDMP)

9.2 Tailoring the Database

9.3 About the Program

9.3.1 Generating Reports

CHAPTER 10: CLASSR00kl EXERCISES

10.1 Purpose

10.2 Exercise Procedure

10.2.1 Exercise 2 Procedure

10.2.2 Exercise 3 Procedure

10.3 Exercise 2

10.3.1 Preliminary Rating

10.3.2 Detailed Rating

10.3.3 Preliminary Design and Cost Estimate

10.4 Exercise 3

10.4.1 Preliminary Rating

10.4.2 Detailed Rating

10.4.3 Preliminary Design and Cost Estimate

10.5 Summaries of Classroom Exercises

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5.2

5.3

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5.8

5.9

5.10

5.11

5.12

5.13

5.14

5.15

5.16

5.17

5.18

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5.20

5.21

5.22

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5.24

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LIST OF FIGURES

The public expects a safe trip.

Railroad accident caused by a rockfall.

"C" rated slope

"B" rated slope

"A" rated slope

Overhanging slope caused by differential erosion.

llO-foot high slope.

Ineffective roadside ditch.

Slope height scoring graph.

Rockfall is generated by the natural slope

above the highway cut.

When the slope is nearly vertical and there is access to

the top, the height can be measured with a tape.

Determine the vertical height of the slope not the slope

length. This 73-foot high slope is scored as 25 points.

The width of this fallout area provides good catchment.

The launch feature midway up the slope adds to the

ineffectiveness of this narrow roadside ditch.

Rockfall from this slope will land on the roadway.

The large volume events possible at this site would

not be retained in the ditch.'

Roadway curves with reduced posted speed limits

are common in rugged terrain. Remember to record

this information.

Example of a horizontal curve that could hide a

rock in the road ahead.

Vertical curves can also restrict sight distance.

The measured sight distance is 330 feet.

Only the width of the paved surface is rated. Note

the loss of the unpaved shoulder in the distance.

On divided highways, only the portion available

to the driver is measured.

The bedding is dipping towards the highway.

Randomly jointed rock.

Note the toppling failures. This, too, is an adverse

joint condition.

Continuous joints with adverse orientation.

An irregular joint.

Note the rough texture of the

exposed surface.

Exposed undulating joint surface.

A planar joint surface.

The surface is macro smooth.

Weathered joint surface.

Note the red colored clay

between the rock surfaces.

Rockfall caused by differential erosion.

Movement of material on an active talus slope is a

Case Two condition.

Layered material can be susceptible to d

erosion.

Oversteepened soil/rock slope.

The soil in the joints is more susceptib

than the resistant rock. The difference

ifferentia .l

'ion

e to eros

is large.

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Figure

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5.31

5.32

8.1

8.2

9.1

9.2

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10.2

10.3

10.4

10.5

10.6

10.7

10.8

10.9

10.10

The cinders at the base of the slope are strongly

affected by freeze/thaw cycles and rain. The

differences are extreme.

Block size used to rate this event.

Either block size or volume could be used here to

obtain a maximum score.

Both the climate and water on the slope will eventu-

ally cause problems at this newly constructed site.

Rockfall History major events

A major event.

Note the large overhang in the distance.

When the large block fell, some of the debris reached

the roadway.

future rockfalls, which will be smaller,

should be retained in the ditch.

Screen print of the main state menu.

Sort menu with a request to sort database by Region,

Highway, and beginning Mile Point.

Ranking menu.

The largest value in the category to be

selected for ranking will be ranked 1.

Data ranked by cost-to-RHRS score.

Cost-to-RHRS data ranked by total score.

Overview of site 2. Slope height?

Ditch effectiveness?

AVR? Note the posted speed limit.

Geologic character?

Block size?

Overview of site 3. Slope height?

Ten-foot ditch is inadequate. Seventy-five percent

of rock reaches the roadway. Ditch Effectiveness?

Decision sight distance? Roadway width?

Geologic character?

Block size?

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LIST OF TABLES

Table

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ODOT's RHRS Costs (7/01,'90)

Description of Work Completed

Time Expenditures

Breakdown of Expenses

Expenditures Based on Percent of Time

Spent for Each Step

Preliminary Rating System

Summary Sheet of The Rockfall Hazard Rating System

Exponent Formulas

Decision Sight Distance

Rockfall Mitigation Techniques

Summary of Classroom Exercises

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In a subsequentpaper (2), Wyllie outlined a more detailed rating

procedure for prioritizing rockfall sites. Wyllies approach

included specific categories for evaluation. The categories were

scored using an exponential scoring system.

These were the two approachesutilized in developing the

prototype for the RHRS. From the earlier work of Brawner and

Wyllie, the idea of grouping sites in accordancewith a

subjective evaluation was adopted as part of the preliminary

rating. From Wyllies later work, the rating sheet format with

categories and the exponential scoring systemwere adopted as

part of the detailed rating. Some of the categories are similar to

Brawners and Wyllies while others are new. All have been

modified based on experiences n developing and applying the

RHRS statewide over the past several years. Detailed

narratives of the rating criteria have been added to promote

consistent understanding and application of the system.

The final phase of RHRS development began in July 1989, when

ODOT was selected to perform the HPR pooled-fund study

entitled the Rockfall Hazard Rating System. Funding support

for the study was provided by the following agencies:

State Highway Departments

1. Arizona 6. New Mexico

2. California 7. Ohio

3.

Idaho 8. Oregon

4. Massachusetts 9. Washington

5. New Hampshire 10. Wyoming

Federal Highway Administration

1. CTIP (Direct Federal)

3. Office of Research

2. Office of Implementation

The goal of the study was to finalize an effective RHRS.

Creation of the system was guided, and its value judged

according to several criteria:

1. Was the system understandable and easy to use?

2. Did the narratives adequately explain the criteria?

3. Could several different raters achieve uniform results?

4. Did the scores adequately reflect the rockfall hazard?

Through full-state implementation, the RHRS was tested at more

than 3,000

sites. The narratives were finalized and forms and

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rating aids were developed. All pertinent information was

documented n the RHRS Users Manual (3). Nationally, the

test results were shared with State Highway Departments through

workshops presented at five regional Geotechnical Conferences

sponsoredby the FHWA.

The ODOTs engineering geology staff spent many hours

designing, testing, and redesigning the Rockfall Hazard Rating

System. Their specialized background and understanding of

rockfall made them uniquely qualified to create and maintain the

RHRS system and database.

1.3 Summary of System Features

The RI-IRS is a process that allows agencies to actively manage

the rock slopes along their highway system. It provides a

rational way for an agency to make informed decisions on where

and how to spend construction funds. The six steps n the

processare summarized below.

1. Slope Inventory - Creating a geographic databaseof

rockfall locations (chapter 2).

2. Preliminary Rating - Grouping the rockfall sites into

three, broad, manageably sized categories as A, B, and C

slopes (chapter 3).

3. Detailed Rating - Prioritizing the identified rockfall sites

from the least to the most hazardous (chapters 4 and 5).

4. Preliminary Design and Cost Estimate - Adding

remediation information to the rockfall database chapter 6).

5. Project Identification and Development - Advancing

rockfall correction projects toward construction (chapter 7).

6. Annual Review and Update - Maintaining the rockfall

database chapter 8).

Note that the RHRS uses wo types of slope ratings: the

preliminary rating performed during the initial slope inventory,

and the detailed rating. The preliminary rating eliminates many

slopes from any further consideration. This staged approach is

the most efficient and cost effective way to implement the RHRS


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and is especially useful where agencieshave responsibility for

many slopes with a broad range of rockfall potential.

1.4 RHRS Benefits

The benefits associatedwith a fully implemented Rockfall

Hazard Rating System fall under three main headings:

1. Knowledge

2. Public Perception

3. Legal Protection

The bottom line of all three is that the necessarystepsare

being taken to understand the problem, and that the agency is

actively dealing with its rockfall problems.

1.4.1 Knowledge

Through implementation of the RHRS, management obtains

detailed information and a uniform process that can help them

make practical decisions on where to allocate money for rock-

slope projects. Until an agency knows the extent of its rockfall-

related problems, it cannot reasonably plan to deal with them.

As in all successfulphuming activities, a thorough understanding

of the problem is required

Oregon DOTs experience with the Rockfall Hazard Rating

System has been favorable. They welcome having quality

information to use in this area of project development. The

agency believes the issue of public safety is being properly

addressedand that greater legal protection is afforded the agency

by having the RHRS in place.

1.4.2 Public Perception

The public has come to realize that many risks are associated

with driving and that most of these risks are the result of human

error. In this respect, rockfall related accidents are out of the

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ordinary. Very few highway accidents receive as much adverse

public attention as one caused by a rockfall. We are expected to

do something about the problem.

To offset this reaction, an agency must demonstrate to the public

that it is not only aware of the rockfall problem but is also

taking prudent steps toward reducing rockfall risks. The RHRS

is now recognized as an effective ,system for dealing with

rockfall. Using such a system demonstrates that the agency is

aware of its safety obligations and is taking a proactive approach

to the issue of rockfall.

1.4.3 Legal Protection

The courts have indicated that it is unreasonable to expect an

agency to have at its disposal enough funds to deal with all

safety related issues at any given time. However, a system must

be in place by which needed safety projects, including rockfall

remediation projects, can be identified and developed as funding

is made available. ODOTs experience has indicated that this

position is legally defensible.

The recently implemented RHRS has not been tested in court to

date. However, Oregon has for many years had a priority list

for developing rockfall construction projects. The sites listed

were those identified as having a history of accidents and/or

excessive maintenance costs. The list generally contained only

about 100 sites, selected not for their rockfall potential but for

their rockfall history. The sites were prioritized on the basis of

a benefit/cost analysis. Even so, because ODOT had a definite,

planned approach to deal with rockfall sites as funds were made

available, litigations brought against the state because of rockfall

were either settled out of court or resulted in findings favorable

to the state. Having a federally recognized, state-of-the-art

process for developing the priority list will serve agencies even

better in this regard.

1.5 Implementation

The RHRS process requires a greater commitment and focus on

the rock slope issue than is commonly the norm for many

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agencies. The needed commitment entails additional working

hours and dollars to train the staff, complete the initial survey,

update the database egularly, and develop remedial programs

aimed at reducing the rockfall risk at the worst sites.

Several stepsare recommended or an agency to successfully

implement the RHRS. These steps can be grouped under the

following two headings:

1. RHRS modification.

2.

Staff Training.

Some customization of the RHRS may be necessary.

In

addition, a properly trained and experienced staff is needed to

perform the slope evaluations and to develop remedial designs.

The associatedcosts will vary, depending on an agencys past

experience in relation to the process, ts staff resources, and the

number of rock slopes it must evaluate.

1.5.1 RHRS Modification

It is understandable that some modifications are inevitabie.

Keep in mind, though, that the RI-IRS is a highly developed

system. Thinking through major modifications and retracing

many of ODOTs and FHWAs efforts will likely prove to be

unnecessary.

Within an implementing agency, a committee comprised of

representatives from the geotechnical, maintenance and project

development sections s helpful in guiding implementation of the

RHRS. This guidance helps to ensure that the finished product

is in the most useable form possible.

Consistency s an important aspect of the standard RHRS.

Any

necessarymodifications should be completed prior to full-scale

implementation. These modifications might include:

Changes to the basic RHRS to accommodate ocal

conditions.

Alterations in one or more of the data collection

forms, graphs or work sheets.

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Changes to the Rockfall DatabaseManagement

Program (RDMP), or development of the agencys

own databasesystem.

Naturally, neither the scope of the rockfall problem nor the

physical conditions that cause t are the same n every agency.

The detailed rating portion of the system evaluates the physical

and historical conditions at a site. The criteria established for

the ratings is based on a wide spectrum of possible conditions.

If the conditions in your area are not covered by the proposed

criteria or if the majority of the slope conditions fall at one end

of the spectrum, then a modification of the criteria is

recommended. This change will allow for adequate score

separation and better identification of the more hazardous sites.

For example, in the slope height category, using the established

criteria, a slope must exceed 105 feet in height in order to

receive the maximum score of 100 points. If very few slopes n

your area are this high, then differentiating the relative risks

associatedwith this category through adequate score separation

will not occur. The criteria should be adjusted to avoid this

problem.

The forms in this manual were created to suit the requirements

of the RI-IRS and the RDMP. However, agencies ypically

differ both in the way they are organized and the way the

document geographic information. Modifying the forms to

conform to an agencys normal procedures may be necessary.

If the scoring criteria are altered, the exponent formulas used to

calculate a score based on the measuredcriteria will also need to

be modified. These formulas have been used to produce scoring

graphs and tables that facilitate the rating process. The scoring

graphs and tables will need to be redeveloped if criteria changes

are made.

A computer database s an important part of the RHRS. A great

deal of information will be generated that must be readily

accessible. In addition, since there are many ways of using the

RI-IRS data, having the flexibility to present the data in several

different formats is important. A hard copy file system will not

meet this need.

The FHWA has developed a Rockfall Database Management

Program (RDMP) that is PC based and designed specifically for

the RHRS. The program is a stand alone database hat requires

no supporting software. A copy is provided at no charge to the

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states. The program operations are quite user friendly. The

RDMP is an excellent tool, and is highly recommended. If

assistance s needed to tailor the RDMP to an agencys needs,

that help is available through the FHWA.

If an agency already has a mainframe databasesystem, it may

want to adapt it to include the RI-IRS information.

Being able to

network through a statewide systemoffers the advantage of

allowing rapid transfer of information.

1.5.2 Training

Successfulcompletion of the RHRS depends on the efforts of

many people. The staff of raters, data entry personnel, and

designers all need training. In Oregons case, all these duties

were performed by its staff of engineering geologists. They

helped develop the RHRS, performed the ratings, entered the

data, and prepared the preliminary designs and cost estimates.

Becauseof proper training, they have since successfully

demonstrated hat reasonable and repeatable slope ratings can be

achieved and quality preliminary designs produced.

The responsibility for slope evaluations and design concepts

should rest with the more experienced staff.

Training should

consist of both a classroom style introduction to the RHRS and

additional hands-on field training beyond what was provided

during the second day of this course. Joint field exercises will

help the group reach a consensuson how to apply the criteria.

Communication within this group during implementation will

help maintain consistent application of the RHRS.

Prior to full-scale implementation, several rockfall sites should

be identified for rating. The sites should be selected to provide

as wide a variety of conditions as possible. Each rater should

rate all of the sections ndependently. A peer review of the

results should be undertaken to insure uniform application of the

criteria. (A smaller group of raters will promote more

consistent, reproducible, and useable results than a larger

group.) It is best to make any final modifications to the RI-IRS

or operational procedures at this time.

If a mainframe system s used, input of the RHRS information

into the databasemay be assigned to data entry personnel. Their

familiarity with data entry can reduce the training cost of this

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effort to nearly zero. However, utilizing the extra staff to

ensure that the data is understandable to these individuals may

not make it the best option. If the raters input the data, some

time will be needed to familiarize them with the procedure.

The

PC based BUMP databasesystem was made as user friendly as

possible which should minimize the required training.

Developing state-of-the-art preliminary designs and cost

estimates s a specialized skill. This function may be outside an

agencys capabilities and may require contracting out. If the

agencys staff is capable in this area only routine review is

required, and little or no training will be necessary. Because

the designs are preliminary, less experienced raters may produce

these design concepts as long as their work is closely reviewed

by a qualified in-house specialist.

The new experience gained

by these raters will be beneficial.

1.5.3 costs

Cost is a primary concern for any agency considering

commitment of their resources o this kind of effort. The costs

associatedwith this commitment are jointly dependant on an

agencys pay scalesand the scope of the rockfall situation. One

way for an agency to estimate its costs s through comparison

with Oregons (in 1990 dollars). The following tables detail

ODGTs implementation costs.

Table 1.1 ODOTS RHRS Costs (7/01/90)

Middle Pay Step

Class (Loaded Rate)

Hours

Charged Total

Geologist 1

Geologist 2

Geologist 3

Supv Geol.

Office Spec.

Eng. Tech. 1

Maim. For. 2

Transp. Eng. 1

Total Overtime

Expenses

$21.81/hr.

$24.Ol/hr.

$27.84/hr.

$30.72&r.

$16.25&.

$16.29/hr.

$17.42/hr.

$24.Ol/hr.

variable

878.5

793.0

164.0

448.0

44.5

139.0

960.0

80.75

66.5

Total $87,169.52

$19,160.08

$19,039.93

$4,565.76

$13,762.56

$ 723.12

$2,264.31

$16,732.00

$ 1,942.ll

$ 1,654.65

%7,325.(M

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Table 1.2 Description of Work Completed

Number of

Total slopes inventoried statewide.

All were

assigned an A,B,C rating.

1340

Slopes that were identified as

A or

B slopes and

were entered into the RHRS database.

501 Slopes that were designated as A slopes and were

further evaluated using the detailed rating.

* Estimated, since C rated slopes were not documented.

Table 1.3 Time Expenditures

Personnel

Involve

All

e

3493.5 hrs. or 436.68 days

Geologists (only)

2283.5 lm. or 285.00 days

Table 1.4 Breakdown of Expenses

All A, B, C cuts

A and B cuts only

A cuts only

Per r&me*

$29.00 spent per cut rated



* Total cost/no. of slopes n grouping.

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Table 1.5 Expenditures Based on Percent of

Time Spent for Each Step

Training 10%

2%

$ 1,743.39

Prelim. Rating 50% 53% $46,199.84

Detailed Rating 30%

31% $27,022.55

Design/Cost Est.

10% 14% $12,203.73

* Suggestedas a guideline for implementing the RI-IRS.

Actual percentageswill vary from agency to agency.

When completed, the final RHRS product is a statewide database

that identifies all A and B rated slopes and includes a

preliminary design and cost estimate for all of the A rated

slopes. Table 1.4 indicates that the expenditure to complete this

estimate, based solely on the number of A rated slopes, is

$174.00 per slope. This cost is quite reasonable.

1.6 Limitations

Agencies will always be expected to react to rockfall accidents,

no matter where the accident area ranks on the RHRS priority

list. The tendency to overreact should be resisted. Sites where

an accident has occurred should be reevaluated using the detailed

rating to determine if the rockfall incident has increased or

decreased he rockfall potential. The level of investment at the

site should be consistent with the newly evaluated rockfall

potential relative to the other sites.

The Rockfall Hazard Rating System offers agencies a method to

prioritize their rockfall problems by providing a relative rating

among slopes. This rating is partially subjective. Although the

slope evaluation process s as straightforward as possible, there

is still a range of values that a particular slope could receive.

This depends to a large degree on the abilities of the raters and

how consistently they interpret and apply the rating criteria.

Keep in mind that any

A

rated slope is capable of sending

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rock onto the roadway, whether it receives a detailed rating

score of 700 or 600 points.

1.7 Conclusion

Oregons experience with the RHRS has been favorable.

The

responseby agency managementhas been one of relief and

acceptance. They welcome having quality information to use in

this area of the project development process. They can now

make rational decisions on where to allocate money for rock

slope projects, a capability that was not possible before

implementation of the Rockfall Hazard Rating System.

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CHAPTER 2: SLOPE SURVEY

2.1 Purpose

The purpose of the slope survey is to gather specific information

on where rockfall sites are located. This first step is an essential

feature of the RHRS. Only through this effort can an agency

understand the extent of the rockfall problem it faces.

2.2 Approach

It is best to approach the survey without preconceived ideas of

the location or number of hazardous sites. Few people will

already be aware of even the most critical rockfall sections

statewide.

The slope survey is an information gathering process hat, in the

beginning, may seemunreasonably burdensome. It is best to

break the effort into manageable asks. Using existing

maintenanceboundaries is a practical starting point.

Accurate delineation of the rockfall section is important.

For

the purpose of the RHRS, a rockfall section is defined as:

Any unint.errupti slope along a highway where the level

and occurring mode of rockfall are the same.

The limits of a rockfall section are established by the length of

the slope adjacent to the highway. Interruptions in the slope

may be due to numerous factors such as drainages, cross oads,

and taposraphy *

Within an uninterrupted section, the level (frequency and/or

amount) of rocl&ll can be markedly different. Two examples

of these differences are a change in the frequency or orientation

of the discontinuities and a difference in erosion rates. Where

this kind of variation occurs, the urgency and type of

remediation required may vary enough to make delineating

portions of the slope as separate ockfall sections appropriate.

The rockfall mode (the reason for the rockfall) may also vary.

Changes n rock type, slope geometry, or the size of the fallout

area can occur throughout an uninterrupted slope section. Thus,

the slope can have different material properties exposed, or

varying conditions that lead to rockfall on the highway. The

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type of remediation will vary frequently becauseof these

variations and delineating additional sections s appropriate.

Establishing rockfall sections n this manner requires both skill

and effort.

The desired result is a flexible and usable database.

Grouping of sites can occur later, if needed, when project limits

are defined during the project development process.

2.3 Personnel

Two people are needed for the slope survey: the rater, who will

perform the preliminary rating and, if needed, the detailed

rating, and a person from highway maintenance. The mainte-

nance person should be the one who is most knowledgeable

about a highways rockfall history and associatedmaintenance

activities.

2.4 Information Gathered

The historic perspective provided by the maintenance person is

an important element of the preliminary rating.

Past rockfall

activity is a good indicator of what to expect in the future. Too

often, the rockfall history of a site is not well documented, and

is maintained only in the memory of the person who worb on

that section of highway. The slope survey provides an

opportunity to document the historic rockfall activity. The

following information should be covered:

1. Location of rockfall activity

2. Frequency of rockfall activity

3. Time of year when activity is highest

4. Size/quantity of rockfall per event

5. Physical characteristicsof rockfall material

6.

Where rockfalls have come to rest

7. Available accident history

8. Opinion of rockfall cause

9. Frequency of ditch cleaning/road patrol

10. Estimated cost of maintenance response

This information should be recorded in the Comments section of

the field data sheet shown on the next page.

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RNRSIELD ATA RRET

REGl;N

PercentDecisionSite

Distance

SIGBT ISTANCE

CASE1

Structural ConditionD C/PR A

Rock riction R I 0 P C - S

CASE

Differential ErosionFeatures 0 N N

Differencen ErosionRatesS N L E

BlockSize/Volume ft/yd3

Climate

CASE

STRUCTCOND

ROCKRICTION

CASE

DIF ERFEATURES

DIP ERRATES

BUCKSIZE

Precipitation L WH

FreezingPeriodN S L

Water n Slope N I C

Rockfall History F 0 N C

amnERTs:

CLIHATE

ROCKFALLISmY

lwrAL SCORE

16


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At the top of the data sheet, the highway name and number, the

region or district, the beginning and ending mile point, whether

the section is left or right of centerline, the county, the date, and

the raters name, should be recorded. The limits of the rockfall

section should be determined to the nearest hundredth of a mile.

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CHAPTER 3: PRELIMINARY RATING

3.1 Purpose

The purpose of the preliminary rating is to group the rockfall

sections nspected during the slope inventory into three broad,

more manageably sized categories. Without this step, many

additional hours would be spent applying the detailed rating at

sites with only a low-to-moderate chance of ever producing a

hazardous condition. This rating is a subjective evaluation of

rockfall potential that requires experienced, insightful personnel

to make valid judgments.

3.2 Criteria

The criteria used in the preliminary rating to categorize sections

as A, B, or C slopes are shown below.

Table 3.1 Preliminaxv Rating System

CLASS

CRITERIA

ESTIMATED POTENTIAL

FOR ROCKFALL

ON ROADWAY

HISTORICAL

ROCKFALL ACTIVITY

A B

C

HIGH MODERATE LOW

HIGH MODERATE LOW

The RI-IRS is a proactive system, primarily aimed at the rockfall

potential at a site. The estimated potential for rockfall on

roadway criterion is therefore the controlling element of the

preliminary rating. For example, if a slope presentsa high

potential for rock on the roadway, (i.e., the slope contains a

large block that displays evidence of active displacement and

very limited fallout area is available), it would receive an A

rating, regardless of other past rockfall activity. The historical

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The slope is over 100 feet tall and it shows the severe effects of

differential erosion (figure 3 S). The roadside ditch is not

adequate to restrict rockfall from reaching the roadway (figure

3.6). The maintenanceperson states hat the site is a major

maintenance problem. Cobble-size rocks from the upper

conglomerate unit fall on the road almost daily from November

to May (rainy period) and during summer storms. In addition,

about every 3 to 5 years a major rockfall event occurs, when too

much of the slope becomesunsupported. Water flowing out of

the slope between the two sedimentary units rapidly erodes the

lower unit, resulting in the overhang.

Based on this evidence, classify the criteria in accordancewith

the preliminary rating system shown on page 18.

Estimated Potential for Rock on Roadway = A B C

Historical Rockfall Activity = A B C

This rockfall section would be entered into the RI-IRS database

as

an A B classified slope.

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CHAPTER 4: DETAILED RATING

4.1 Purpose

The purpose of the detailed rating is to numerically differentiate

the risk at the identified sites. Once rated, the sites can be

sorted and prioritized on the basis of their scores. These lists

are then used to help make decisions on where safety projects

should be initiated.

4.2 Overview

The detailed rating, shown on the next page, includes 12

categories by which slopes are evaluated and scored. (A

detailed explanation of these categories is included in chapter 5.)

The category scoresare then totaled. Slopes with higher scores

present the greater risk. These 12 categories represent the

significant elements of a rockfall section that contribute to the

overall hazard. The four columns of benchmark criteria to the

right correspond to logical breaks in the increasing risk

associatedwith each category.

4.3 Scoring System

Accordingly, as the risk increases rom left to right, the related

scoresabove each column increase from 3 to 8 1 points. These

set scores ncrease exponentially. An exponential scoring system

provides a rapid increase in score that distinguishes the more

hazardous sites. The set scoresare merely representatives of a

continuum of points from 1 to 100. When rating a slope, using

the full range of points instead of only the set points listed above

each column allows the rater greater flexibility in evaluating the

relative impact of conditions that are extremely variable.

Initially, novice raters feel more comfortable using the set

points. Less udgement is required. Continuing to use only the

set points, however, is not an optimal use of the system, and is

not recommended.

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SHEET OF THE ROCKFALL HAZARD RATING SYSTEM

AVERAGEEHICLE

PERCENTF

DECISION

SIGBT

DISTANCE

ROADNAYIDTH

INCLDDINGAVFJ

sJlmERs

Adeguateight

distance,004

of lowdesign

value

44 eet

b&rate sight

distance,09

of lowdesign

value

36 eet

Limited ight

distance,01

of lowdesign

value

28 eet

Veryimitedsight

distance0%

of lowdesign

value

20 eet

G C Discontinuous Discontinuous Discontinuous Continuous

E A STRUCTURAL joints,

joints,

joints,

joints,

0 s CDNDITIDN favorable randon

adverse adverse

L E orientation orientation

orientation orientation

0

G 1

ROCK

Rough,

Claynf lling,

I

FRICTIDN

Irregular Undulating Planar or slickensided

C

c c

A

Few

Occasional

WY

Hajor

STRUCTURAL

A s

differential differential differential

differential

R E

CONDITION erosioneatures erosion erosion erosion

A

features features features

c 2

T

E

DIFFERENCEN Small Hoderate

Large

Extreme

R

EROSIONATES difference

difference difference difference

BLOCKHE

VGF

RocKFALL/AmT

CLINATEND

PRESENCE

OFWATER

ON LOPE

1 Foot

3cubic

Yards

Lowto

aoderate

precipitation:

no reezing

periods:o

water nslope

2Feet

6cubic

Yards

H&rate

precipitation

or short reezing

periodsr

inter&tent

water nslope

3 Feet

g-ii5

Y&

High recipitationr

long reeaing

periodsr

continual ater

onslope

4 Feet

12cubic

Y-

Bigh recipita-

tion andong

freeaingeriods

or continual

water nslope nd

long reezing

periods

ROCNFALLISTORY

Fewalls Occasionalalls Nanyalls Constantalls

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4.4 Scoring Aids

To assistwith scoring, scoring graphs have been created for all

categories, and scoring tables have been developed for all of the

directly measurablecategories. These scoring aids promote

greater consistency in assigning scores, and increase the speed of

performing the detailed rating. The graphs are useful even for

the subjective categories, especially if previously rated slope

conditions are plotted directly on the graphs as a reference.

4.4.1 Graphs

The graphs relate the category evaluation to an appropriate

score. Even with the subjective categories, such as Ditch

Effectiveness, the graphs are quite useful in assigning a score to

a condition that falls somewherebetween the described

benchmarks.

The curve on the graph is the plot of the function

Y

= 3 which defines the exponential scoring system used for

all categories. These graphs are designed to match the

established criteria. If modifications to the criteria are made,

the graphs must be corrected to match the new criteria.

1

25 50 75 100

Slope Height (ft)

Figure 4.1 Slope height scoring graph.

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4.4.2 Exponent Formula

Exact scorescan be tabulated for the measurable categories by

calculating the value of the exponent xl of the function y = 3.

The formulas that yield the exponent values are included in the

table below. Some agenciesmay prefer to include these

formulas in their databaseso that only the sites measurements

need to be entered. Remember, these formulas will need to be

modified if the rating criteria are changed.

Table 4.2 Exponent Formulas

SLOPE HElGKT ROADWAY WIDTH

X

Slopa Ht.@.) 52 - Roadway Width (ft.)

=

X

=

25 B

AVERAGE VE WLE RISK

I

BLOCK SIZE

X Time

x=

X

= Block Size (ft.)

25

SIGHT DISTANCE VOLUME

x=

120 - X Decision Sight D ist

x=

Volume (cu.R.)

20

3

4.4.3 Scoring Tables

Scoring tables have been produced using these exponent

formulas. Scores derived from these tables are always

reproducible. The only variations possible are those due to

human error or to a difference in the category measurement.

The scoring tables shown on the next page, along with useful

formulas used in the detailed rating, are included on the back of

each field data sheet.

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SLOPE

HT.

u6r6

56 13

EEi

'9 13

59 13

60 14

61 15

375

2s

30

CL? 5 35

40

E

75

45

750 50

675

55

looo 60

lael 90 llnl 11 I

2 2

2.5 2

Elii

3

3.5 4

4 4

39 06 1 78 10

40 at 79 10

41 n 8a 9

42 .73 81

9

62 is

63 18

ij

64 17

% 17

143169 i62l 6 1

I

4.5 I 8 I

1

8

71

l-z-ma

27

l-t

lag, 3 I

61

1 13 1 loo I

67 46

66 46

33

99 so

90

52

lJst6 iaS

92 57

93 60

33

94 60

95 62

96 65

100

7

1241 xsiumleMlh

hike x

lul

7

Ys.~I-601

97

%

4

99

76

loo a1

33

01 %

102 66

103 92

E.P.

I

lope Height =

sina xsinfi XX

sin(a - 6)

+ H.I.

-1

Heightof Instrument

a

and/3

I

hottzontaldistancebetween

I

I

I DITCH HIGHWAY

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5.1 Category Narratives

Before a decision can be made on how to score a rockfall section

the criteria for each category must be well understood and

carefully considered. As the RI-IRS evolved, it became clear

that the scoring criteria needed more clarification to limit the

degree to which the raters could interpret the criteria differently.

To improve rating consistency, narratives were written for each

category.

The narratives are based on extensive field testing of the system.

Some categories require a subjective evaluation, while others can

be directly measured and then scored. The narratives describe

the benchmark criteria in greater detail. This description

reduces the possible variation in scoring a category by limiting

the amount of interpretation required by the rater.

5 -2 Category Photographs

Photographs from several sites will be used as illustrations of the

category criteria. The photographic examples are valuable aids

for relating the criteria to actual slope conditions and, in some

cases, for demonstrating the intended limits of the criteria.

They

will be beneficial references should you consider possible

modifications to the criteria.

Classroom exercise 1, used as the preliminary rating example,

will also be used throughout this chapter. This exercise will be

the participants first opportunity to apply the detailed rating to a

slope. Photographs showing the condition to be rated will be

included at the end of each category discussion.

5.3 Slope Height Category

This category evaluates the risk associated with the height of a

slope. The height measured is the vertical height, not the slope

distance. The slope height measurement is to the highest point

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from which rockfall is expected.

If rockfall is generated from

the natural slope above the cut slope, the measurement should

include both the cut height and the additional vertical height on

the natural slope to the rockfall source.

The benchmark heights

that coincide with the set points are listed below.

This category

is directly measured and scored.

SLOPE HEIGHT 25 ft

50 ft 75 ft 100ft


The higher a rock is located on a slope, the more potential ener-

gy it has. The increased energy potential is a greater hazard,

and thus a higher rating is given as the slope height increases.


The slope height can be obtained by using the relationship shown

below.

SLOPE HEIGHT DIAGRAM

TOTAL SLOPE HElGHT =

(X) sin a .

sin p

+ H.I.

(1)

sin( a - B )

Where: X = distance between angle measurements.

H.I. -

height of the instrument.

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Determine the height of the slope by using the method outlined

in section 5.3 -2. The measurements and related facts) were:

1. The clinometer was held at a height of 5 feet (H.I.).

2. The X distance from edge of pavement to edge of pavement

was 30 feet.

3. The Q and B angles measuredwere 70 and 57 degrees,

respectively.

Total Slope Height is

feet.

Using the scoring table in section 4.4.3 (page 30)) the Slope

Height Category score is .

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5.4 Ditch Effectiveness Category

The effectiveness of a ditch is measuredby its ability to restrict

falling rock from reaching the roadway. Many factors must be

considered n evaluating this category. The reliability of the

result depends heavily on the raters experience. Ditch

Effectiveness is a subjective category. The benchmark criteria

are shown below.

DITCH

Good Moderate

Limited

No

EFFECTIVENESS catchment cat&rent catchment catchment

5 -4.1 Category Significance

The risk associatedwith a particular rock slope section is

dependent on how well the ditch is performing in capturing

rockfall. When little rock reaches the roadway, no matter how

much rockfall is released from the slope, the danger to the

public is low and the score assesseds low. Conversely, if

rockfall events are rare occurrencesbut the ditch is nonexistent,

the resulting hazard is greater and a higher score is assigned his

category.

5.4.2 Category Measurement

A wide fallout area does not necessarily guarantee that rockfall

will be restricted from the highway. In estimating the ditch

effectiveness, the rater should consider several factors, such as:

1) slope height and angle; 2) ditch width, depth and shape; 3)

anticipated volume of rockfall per event; and 4) impact of slope

irregularities (launching features) on falling rocks. Evaluating

the effect of slope irregularities is especially important because

they can completely negate the benefits expected from a fallout

area. Valuable information on ditch performance can be

obtained from maintenance personnel.

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5.5 Average Vehicle Risk (AVR) Category

With the AVR category, the risk associatedwith the percentage

of time a vehicle is present in the rockfall section is evaluated.

The percentage s obtained by using the formula (shown below)

based on slope length, average daily traffic (ADT), and the

posted speed imit at the site.

ADT (cars/day) X Slope Length (miles) / 24 (hours/day)

XlOO%=AVR

Posted Speed Limit (miles/hour)

The results are rated based on the established benchmark criteria.

AVERAGE VEHICLE 25%

RISK of the

WR)

time

50%

of the

time

75%

of the

time

100%

of the

time


Combining the ADT, the length of the rockfall section and the

posted speed imit produces a category that represents the

potential for a vehicle to be involved in a rockfall event. The

average percent of time a vehicle is present within the rockfall

section is calculated. Another way of looking at this is that it

shows how many vehicles are in the rockfall section at any one

time. A rating of 100% means hat on the average a vehicle

will be within the defined rockfall section 100% of the time.

Where high ADTs or longer slope lengths exist, values greater

than 100% will result. When this occurs, it means that at any

particular time, more than one vehicle is present within the

measured section. The result approximates the likelihood of

vehicles being present and thus involved in a rockfall incident.

The result also reflects the significance of the route.

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5.6 Percent of Decision Sight Distance Category

The Decision Sight Distance category compares the amount of

sight distance available through a rockfall section to the low

design amount prescribed by AASHTO. Sight distance is the

shortest distance that a six-inch object is continuously visible to

a driver along a roadway. Decision sight distance (DSD) is the

length of roadway, in feet, required by a driver to perceive a

problem and then bring a vehicle to a stop.

PERCENT OF Adequate sight Moderate sight

Limited sight

Very limited

DECISION distance, 100% distance, 80 % distance, 60% sight distance,

SIGHT of low design of low design

of low design 40% of low

DISTANCE value value value design value


The DSD is critical when obstacles on the road are difficult to

see, or when unexpected or unusual maneuvers are required.

Throughout a rockfall section the sight distance can change

appreciably. Horizontal and vertical highway curves along with

obstructions such as rock outcrops and roadside vegetation can

severely limit a drivers ability to notice and react to a rock in

the road.


First, record the posted speed limit throughout the rockfall

section.

Then drive through the site from both directions to

determine where the sight distance is most restricted.

Decide

which direction has the shortest line of sight. Both horizontal

and vertical sight distances should be evaluated. Normally an

object will be most obscured when it is located just beyond the

sharpest part of a curve.

Place a six-inch object in that position

on the fogline or on the edge of pavement if there is no fogline.

Then walk along the fogline (edge of pavement) in the opposite

direction to the traffic flow, measuring the distance it takes for

the object to disappear at an eye height of 3.5 ft above the road

surface. A roller tape is helpful for making this measurement.

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5.6.3 AASHTO Decision Sight Distkes

The required decision sight distance, based on the posted speed

limit can be determined from the table below.

Table 5.1 Decision Sight Distance

Posted Speed Limit (mph)

25

30

35

40

45

50

55

60

65

Decision Sight

Distance (ft)

375

450

525

600

675

750

875

1,~

1,050

The relationships between decision sight distance and the posted

speed limit were modified from table III-3 of AASHTOs

Policy on Geometric Design of Highways and Streets (4).

The distances listed represent the low design value. The posted

speed imit throughout the rockfall section should be used

instead of the highway design speed.

5.6.4 DSD Formula

Once the actual sight distance is measured and the recommended

sight distance determined from the table, the two values can be

substituted into the following formula to calculate the Percent

of Decision Sight Distance.

Actual Sight Distance

x 100% = %

Decision Sight Distance

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5.7 Roadway Width Category

The roadway width is measured perpendicular to the highway.

The minimum width throughout the rockfall section is used when

the roadway width is not constant. The unpaved shoulder

adjacent to the roadway is not included in the measurement.

The benchmark criteria are:

ROADWAY WIDTH

INCLUDING PAVED 44 feet 36 feet 28 feet 20 feet

SHOULDERS

L


If a driver notices rocks in the road, or rocks falling, it is

possible for the driver to react and take evasive action to avoid

them. The more room there is for this maneuver, the greater

the likelihood the driver will successfully miss the rock without

hitting some other roadside hazard or oncoming vehicle. The

measurement represents the available maneuvering width of the

roadway.


The roadway width is measured perpendicular to the highway

centerline. The edges of pavement define the roadway. It is

difficult to get uniform estimates among different raters about

what is unpaved shoulder and what is unmaneuverable side

slope. For that reason, the unpaved shoulders are not included

in the measurement. When the roadway width varies throughout

the rockfaIl section, the section measured should be the area of

minimum width. On divided roadways, only the portion

available to the driver is measured.

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The road through this section consists of two 1Zfoot travel

lanes, a 4-foot paved shoulder on the south side of the road, and

a 2-foot paved shoulder on the north side near the slope.

According to the appropriate scoring table, what is the Roadway

Width Category score for this site?

Roadway Width Score =

5.8 Geologic Character

The geologic conditions of the rockfall section are evaluated

with these categories. Since the conditions that cause rockfall

generally fit into 2 categories, Case One and Case Two rating

criteria have been developed. Case One is for slopes where

joints, bedding planes, or other discontinuities, are the dominant

structural features that lead to rockfall.

Case Two is for slopes

where differential erosion or oversteepening is the dominant

condition that controls rockfall.

Whichever case best fits the slope should be used for the rating.

If both situations are present, and it is unclear which dominates,

both are scored, but only the worst case (highest score) is used

in the rating. The criteria for the two cases are shown below.

c

STRUCTURAL

Discontinuous Discontinuous Discontinuous Continuous

A

CONDITION

joints, joints, joints, joints,

S favorable random adverse adverse rientation

E orientation orientation orientation

G C

EH 1

ROCKRICTION Rough, Undulating Planar Clay nfilling,

0 A

Irregular or slickensided

L R

0 A

G C

I T

CE c

STRUCTURAL Few ifferential Occasional Hanyerosion Haor erosion

R i

CONDITION erosion eatures erosion eatures features features

E

DIFFERENCE

%a11 Moderate

Large

Large

IN difference difference

2

difference, difference,

EROSION favorable unfavorable

RATES structure structures

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5.8.2 Case One, Rock Friction Category

The potential for rockfall by movement along discontinuities is

controlled by the condition of the joints. The condition of the

joints is described in terms of micro and macro roughness.

The

roughness is rated on the basis of the following criteria.

ROCK Rough,

FRICTION Irregular

Undulating Planar Clay infilling

or slickensided


This parameter directly affects the potential for a block to move

relative to another. Friction along a joint, bedding plane, or

other discontinuity is governed by the macro and micro

roughness of the surfaces.

Macro roughness is the degree of

undulation of the joint relative to the direction of possible

movement. Micro roughness is the texture of the surface. On

slopes where the joints contain hydrothermally altered or

weathered material, movement has occurred causing slickensides

or fault gouge to form, or the joints are open or filled with

water, the rockfall potential is greater.


Following are the benchmark criteria descriptions.

3 points &Q& Irrea The surface of the joints are rough

and the joint planes are irregular enough to cause

interlocking.

9 points

m Macro rough but without the

interlocking ability.

27 points planar Macro smooth and micro rough joint

surfaces. Friction is derived strictly from the rough-

ness of the rock surface.

. . .

81points moor- Low friction materials

separate the rock surfaces, negating any micro or

macro roughness of the joint surfaces.

Slickensided

joints also have a lower friction angle, and belong in

this category.

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5.8.3 Case Two, Structural Condition Category

This case is used for slopes where differential erosion or

oversteepening is the dominant condition that leads to rockfall.

Erosion features include oversteepened slopes, unsupported rock

units (overhangs), or exposed resistant rocks on a slope, which

may eventually lead to a rockfall event. The benchmark criteria

are:

I

TRUCTURAL Few differential Occasional

I

Many

Major

CONDITION erosion features erosion features erosion features erosion features


Rockfall is commonly caused by erosion that leads to a loss of

support either locally or throughout a slope. The types of slopes

that may be susceptible to this condition are: layered units

containing more easily erodible units that undermine more

durable rock; talus slopes; highly variable units, such as

conglomerates, and mudflows, that weather differentially,

allowing resistant rocks and blocks to fall; and rock/soil slopes

that weather allowing rocks to fall as the soil matrix material is

eroded.


Scoring should be consistent with the following criteria

descriptions.

3 points

pew Dim Erosiom Minor differential

erosion features that are not distributed throughout

the slope.

9 points Occ~ Fro&n Fe- Minor differential

erosion features that are widely distributed

throughout the slope.

27 points

.

&&my Erom Featurs Differential erosion features

that are large and numerous throughout the slope.

81 points Major Erosion Fm Severe cases such as

dangerous erosion-created overhangs, or significantly

oversteepened soil/rock slopes or talus slopes.

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5 -8.4 Case Two, Difference in Erosion Rates Category

The materials comprised in a slope can have markedly different

characteristics that control how rapidly weathering and erosion

occur. As erosion progresses, resulting in portions of the slope

becoming unsupported, the likelihood of a rockfall event

increases. The benchmark criteria listed below relate to the

difference in erosion,rates within a slope and how they affect the

risk of rockfall.

DIFFERENCE

IN EROSION

RATES

Small Moderate

difference difference

Law

difference,

favorable

structure

Law

difference,

unfavorable

structure


The rate of erosion on a Case Two slope directly relates to the

potential for a future rockfall event. As erosion progresses,

unsupported or oversteepened slope conditions develop. The

impact of the common physical and chemical erosion processes,

as well as the effects of mans actions, should be considered.

The degree of hazard caused by erosion and thus the score given

this category, should reflect the rate at which erosion is

occurring; the size of rocks, blocks, or units being exposed; the

frequency of rockfall events; and the amount of material released

during an event.


Scoring should be consistent with the following criteria

descriptions.

3 points Small Differens Erosion features take many years

to develop. Slopes that are near equilibrium with

their environment are covered by this category.

9 points Moderate The difference in erosion rates

allows erosion features to develop over a period of a

few years.

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If both situations are present, both are scored but only the worst

case (highest total Case One or Case Two score) is used in

determining the rating. If a localized area is causing a serious

rockfall problem, the overall length of the slope being rated

should be reduced, and the problem area should be rated

separately.


The upper portion of the slope is a conglomerate unit with hard

quart&e and igneous rock cobbles suspended n a cemented sand

matrix. About 10 to 15 feet from the base of the slope is a near

horizontal contact between the conglomerate.unit and a poorly

cemented siltstone unit. Water flows from the slope year round

at the contact between the two units.

Two types of rockfall occur at this site, both causedby the same

geologic process - differential erosion. The difference in erosion

rates between the conglomerate and the siltstone units is

significant. The water daylighting in the slope at the contact

between these units accelerates he erosion of the siltstone

creating dangerous overhangs. The size of the overhang

increasesby several inches per year, until the overhanging

material breaks off in a major rockfall event. The failures occur

along vertical stress elief joints that form in the conglomerate

unit parallel to the slope face. These are the major events that

the maintenanceperson described as happening approximately

every 3 to 5 years. While this process s taking place, the

abundant ram and wind at the site erodes the matrix material of

the conglomerate causing the cobbles to fall on a regular basis.

The historic information combined with the knowledge of how

the geologic processes esult in the rockfall events, leads to the

conclusion that this is a Case Two slope.

Using the above information and the narratives for the

benchmark criteria found on pages 55 and 58, assign a score for

the two categories under Geologic Character, Case Two.

Structural Condition Score =

Difference in Erosion Rates Score =

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5.9 Block Size or Volume of Rockfall Per Event Category

In some rockfall events, the failure is comprised of an individual

block. In others cases,. he event may include many blocks of

differing sizes. Which ever type of event is typical is rated

according to the following category benchmarks.

BLOCK SIZE

Ifi

QUANTITY OF 3 cubic

ROCKFALL/EVENT

yards

2ft

6 cubic

yards

3ft

9 cubic

yards

4ft

12 cubic

yards


Larger blocks or volumes of falling rock produce more total

kinetic energy and greater impact force than smaller events. In

addition, the larger events obstruct more of the roadway

reducing the possibility of safely avoiding the rock(s).

In either

case, the larger the blocks or volume the greater the hazard

created and thus the higher the assigned score.

5.9.2 Which Criterion to Use

This measurement should be representative of the type of

rockfall event most likely to occur. If individual blocks are

typical of the rockfall, block size should be used for scoring.

If

a mass of blocks tends to be the dominant type of rockfall,

volume per event should be used. A decision on which to use

can be determined from the maintenance history, or estimated

from observed conditions when no history is available. This

measurement will also be beneficial in determining remedial

measures.

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During the preliminary rating, the maintenance person explained

that this site has two different types of rockfall, both of which

occur on a regular basis.

One type consistsmostly of 3-to-5

inch cobbles that erode out of the conglomerate unit.

Occasionally, portions of the conglomerate up to about 3 feet in

diameter break off. The second ype of rockfall occurs when the

large overhang becomesunstable, and a large volume of rock

falls off at one time.

The volume of these singular events

usually exceeds50 cubic yards of material on the road.

Using the scoring tables, rate the two types of rockfall that occur

at the site, and determine which score to use for rating this

category.

Block Size Score =

Quantity of Rockfall/Event Score =

Score for this category =

5.10 Climate and Presence of Water on Slope Category

The effects of precipitation, freeze/thaw cycles, and water

flowing on the slope are evaluated with this category according

to the following benchmark criteria.

CLIMATE

AND

PRESENCE

OF WATER

ON SLOPE

LOWtO

Moderate

moderate

precipitation or

precipitation;

short freezing

no freezing periods; or

periods; no , intermittent water

, water on slope

I

on slo@e

High precipitation

or long freezing

periods; or

continual water on

slope

High precipitation

and long freezing

periods; or

continual water on

slope and long

, freezing periods

To assureproper score separation, the criteria for this category

should be adjusted to fit local conditions.

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Water and freeze/thaw cycles both contribute to the weathering

and movement of rock materials and a reduction in overall slope

stability. This category evaluates the amount of precipitation

and duration of freezing periods, because hese are measurable

quantities that are directly related to features that cause ockfall.

In addition, water flowing on a slope promotes erosion and thus

is also considered in this category.

5.10.2 Method of Evaluation

If water is known to flow continually or intermittently from the

slope, it is rated accordingly. Areas receiving less than 20

inches per year are low precipitation areas. Areas receiving

more than 50 inches per year are considered high precipitation

areas. The impact of freeze/thaw cycles can be estimated from

knowledge of freezing conditions and their effects at the site.

The rater should note that the 27-point category is for sites with

long freezing periods or water problems such as high

precipitation or continually flowing water. The 81-point

category is reserved for sites that have both long

freezing

periods and one

of the two extreme water conditions.

5.10.3 Information Source

Information on average temperatures and length of freezing

periods can be obtained from National Oceanic and Atmospheric

Administration (NOAA) climatological publications (5). An

agency-wide precipitation map is a useful scoring aid. This

information is usually available from routinely maintained, state-

wide rain gauge data.

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5.11 Rockfall History Category

This category rates the historical rockfall activity at a site as an

indicator of future rockfall events. Typically, the frequency and

magnitude of past events is an excellent indicator of the type of

events to expect. The benchmark criteria established for this

category are:

ROCKFALL

HISTORY

Few falls Occasional falls

Many falls Constant

falls


The rockfall history directly represents the known rockfall

activity at the site. This information is an important check on

the potential for future rockfalls. If the score you give a section

does not compare with the rockfall history, a review of the

rating is advisable.


3 points

9 points

27 points

81 points

Few Falls Rockfalls occur only a few times a year

(or less), or only during severe storms. This

category is also used if no rockfall history data is

available.

Occasional F& Rockfall occurs regularly.

Rockfall can be expected several times per year and

during most storms.

&Qny FU Typically, rockfall occurs frequently

during a certain season, such as the winter or spring

wet period, or the winter freeze/thaw, etc. This

category is for sites where frequent rockfalls occur

during a certain season but are not a significant

problem during the rest of the year. This category

may also be used where severe rockfall events have

occurred.

Rockfalls occur frequently throughoutonstant Falls

the year. This category is also for sites where severe

rockfall events are common.

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CHAPTER 6: PRELIMINARY DESIGN AND COST ESTIMATE

It is important, when planning highway construction projects, to

establish the desired result.

The desired result is what

determines such things as the project limits, the estimated

construction costs, and the right-of-way needs. Trying to

retrofit a different, more appropriate rockfall design after these

factors have been established is frustrating, at best, and can be

completely impossible.

The fourth step of the RHRS process accounts for this need by

requiring that a preliminary design and cost estimate be included

as part of the RHRS database. This information is used in the

final phase of the prioritization process, when project limits are

established and projects are advanced for construction.

6.1 Approach to Rockfall Control

During the detailed rating, the raters should gather enough site-

specific information to be able to recommend which rockfall

remediation measures are appropriate for the site. More than

one design approach will likely be needed for each site.

For management to make informed decisions on whether to take

a total correction approach or only reduce the rockfall hazard,

they will need to understand the costs and benefits associated

with both choices. Hazard reduction measures can vary from

limited duration improvements such as slope scaling, to more

aggressive steps, such as installing slope screening.

Frequently, a combination of several techniques will work best.

At this early stage, the goal is to provide an appropriate method

to deal with the rockfall problem. These preliminary concepts

can later be refined by more detailed investigation and analysis.

6.2 Rockfall Remediation Designs

Covering the field of rock mechanics is beyond the scope of this

manual. An excellent reference on the subject is the Federal

Highway Administrations publication No. FHWA-TS-89445,

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titled Rock Slopes: Design, Excavation, Stabilization,

prepared by Golder and Associates, Seattle, Washington.

The value of having personnel skilled in this area can not be

stressed oo much. Experience is the best predictor of the

effectiveness of a rockfall remedial design. There are too many

gaps in our understanding of the mechanical properties of rock

masseso rely wholly on an analytical approach.

6.2.1 Common Rdckfail Remediation Techniques

Several techniques are routinely used to deal with rockfall. The

choice of techniques s dependent on several factors, including

the size or volume of anticipated rockfall, access o the rockfall

source, maintenance imitations, the construction budget, and the

desired result. The following table describescommon rockfall

mitigation techniques and their uses.

Table 6.1 Rockfall Mitigation Techniques

Technique

Scaling

Slope Screening

Catch Fences

Excavation

Artificial

Reinforcement

Description/Purpose

Removal of loose rock from slope by means of hand tools

and mechanical equipment. Commonly used in

conjunction with most other design elements.

Placement of wire or cable mesh on a slope face. Controls

the descent of falling rock. Rockfall accumulatesnear the

base of the slope for removal.

Wire or cable mesh draped from a fence to the roadside

ditch. The fence (impact section) captures the falling rock

and channels t beneath the mesh. The mesh attenuates he

rockfall energy, allowing the rock to come to rest short of

the roadway, in the catchment area.

Removal of slope material in order to create a rock fallout

area adjacent to the roadway. Use of modern construction

practices improves the condition of redeveloped cut slopes.

Improvement of slope stability by the installation of

mechanical supports including rock bolts, rock dowels, and

cable lashing. Used to hold material in place on the slope.

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Shotcrete Mortar or concrete pneumatically projected at high velocity

onto a slope.

Primarily used to halt the effects of erosion

by protecting the shot surface from the elements. Also

helps retain rock on the slope.

Barrier Systems Installation of either rigid or flexible barriers systems

capable of handling the energy developed in a falling rock.

Examples include Jersey barriers, gabion baskets and

woven cable fences. Systemsare normally placed adjacent

to the roadway for easeof maintenance.

Drainage

Reduction of the water level within a slope through

installation of horizontal drains or adits. Commonly used

in conjunction with other design elements.

6.2.2 Reviewing Preliminary Designs

Inexperience can result in the wrong application of the above

techniques.

A review of the design concept by an experienced

staff member is recommended before the cost estimate is

calculated, or used to make decisions on project development.

6.3 Cost Estimate

The cost estimate is an important element of the rockfall

database. This information will be considered when final project

priorities are established. The costs of these different design

elements can vary a great deal nationally. For that reason, no

costsare included in this document.

The rockfall design cost calculated is strictly the cost of the

rockfall remedial measures. A project may eventually include

pavement widening, guardrail installation, structural pavement

overlay, etc. These cost items, as well as typical mobilization,

engineering, and contingency costs are not included as part of

the RHRS cost estimate. This approach simplifies the estimation

process, since, when rockfall sites and the costsof dealing with

them are compared, these additional cost items will not interfere.

A sample worksheet is shown on the next page.

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Page of

-

Design Option

ROCKPALL MITIGATION COST ESTIMATE WORKSHEET

State Hwy Name:

Beginning M. P.

L or R of Centerline

Ending M. P.

County Name

Date of Rating(YYMMDD)

Name of Designer

#

#

. Average Daily Traffic

Posted Speed Limit

RHRS Score

GN OPTION D-ION

This approach is a rockfall correction

hazard reduction design.

DESIGN ELEMENTSESIGN ELEMENTS

1..

2..

3..

4..

COST P.ST-

OST P.ST-

Quantity X Unit Costuantity X Unit Cost

1..

X

2..

X

3. X

4. X

5. X

6.

X

7. X

8.

X

TOTAL OPTION COST:

5.

6.

7.

8.

$

$

Cost/RHRS Score Ratio

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This work sheet is helpful in developing a cost estimate.

The

information on the sheet documents the location of the rockfall,

a narrative of the design concept, a list of the design elements, a

list of the costs of the design elements, the total option cost, and

the cost-to-RHRS score ratio. An agency may need to modify

this sheet to meet its specific needs.

6.4 Classroom Exercise 1

A preliminary design and cost estimate for site 1 is included below.

Note that the cost

estimate includes only the cost of the rockfall remediation d

009767 rock hazard rating sys

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